Producing hydrocarbon fluid from a layer of oil sand

ABSTRACT

A method of producing hydrocarbons from a layer of oil sand located in a formation comprises creating a plurality of boreholes in the formation, including a first borehole and a second borehole spaced from the first borehole in a selected direction along which the layer of oil sand extends, and creating a cavity in the layer, the cavity being in fluid communication with the first borehole; extending the cavity in the selected direction by operating fluid jetting means via the first borehole to jet a stream of fluid against the cavity wall; when the cavity is in fluid communication with the second borehole, operating the fluid jetting means via the second borehole to jet a stream of fluid against the cavity wall so as to further extend the cavity; and transporting a slurry of fluid and oil sand from the cavity to a processing facility.

In the industry of hydrocarbon fluid production from subterraneanreservoirs, it is conventional practice that oil is produced fromwellbores by virtue of the high fluid pressures existing downhole. Incase of high viscosity oil, downhole pumps can be applied to pump theoil to surface, or other methods can be applied to increase the oilproduction rate such as steam injection or CO₂ injection into theformation. However, the conventional methods are not adequate for theproduction of bituminous oil such as occurring in the oil sandreservoirs in Canada. As some oil sand layers occur at relativelyshallow depths, typically between 0 to 200 meters, it is common practiceto produce oil from these layers by surface mining whereby theoverburden layer is removed using draglines and/or shovels and trucks.The produced oil sand is transported to one or more processingfacilities for separation of hydrocarbon fluid from the sand slurries.However, for oil sand layers at greater depths, removal of theoverburden is costly and has a significant impact on the environment.Therefore alternative methods for producing oil sands have beenproposed.

One such alternative method is disclosed in a technical paper publishedin CIM magazine by the Canadian Institute of Mining & Metallurgy, 2001,Vol. 94, Nr. 1054, pages 63-66, entitled “Hydraulic underground miningof oil sands—the next big step”. This publication discloses a method ofproducing hydrocarbon fluid from an oil sand layer located in an earthformation, wherein a discharge borehole is drilled into the oil sandlayer and a fluid jetting device is operated to excavate the oil sandlayer and thereby form a cavity in the oil sand layer, wherein a slurryof fluid and oil sand is formed in the cavity as a result of the fluidjetting operation. The produced slurry is transported via the dischargeborehole to a processing facility for processing the slurry.

A different method is disclosed in WO 2007/050180, wherein a subsurfaceformation comprising heavy oil and solids is accessed via verticalinjection and production boreholes, pressurized sufficiently to relieveoverburden pressure, and wherein solids and heavy oil are mobilized andcaused to flow through one of the vertical boreholes by means of varyingdifferential pressure between the injection and production boreholes.Optionally water jetting can be used in a short transitional step andused intermittently or for short periods of time, to locally improve theflow of slurry towards the production wellbore.

However, there is still a need for an improved method of producinghydrocarbon fluid from an oil sand layer.

In accordance with the invention there is provided a method of producinghydrocarbon fluid from a layer of oil sand located in an earthformation, the method comprising:

creating a plurality of boreholes in the earth formation, including afirst injection borehole and a second injection borehole spaced from thefirst injection borehole in a selected direction in which the layer ofoil sand extends, and creating a cavity in the layer of oil sand, thecavity being in fluid communication with the first injection borehole;

extending the cavity in the selected direction by operating fluidjetting means via the first injection borehole to jet a stream of fluidagainst a wall of the cavity;

when the cavity is in fluid communication with the second injectionborehole, operating the fluid jetting means via the second injectionborehole to jet a stream of fluid against the wall of the cavity so asto further extend the cavity; and

transporting a slurry of fluid and oil sand, resulting from operation ofthe fluid jetting means, from the cavity via a discharge borehole to aprocessing facility for processing the slurry, the discharge boreholehaving a lower section extending in the selected direction and being influid communication with the cavity at said wall of the cavity.

In this manner it is achieved that the fluid jetting means is close tothe cavity wall being excavated throughout the fluid jetting operation,so that the fluid jet always impacts the cavity wall with great force.

Suitably the step of further operating the fluid jetting means comprisesremoving the fluid jetting means from the first injection borehole andinserting the fluid jetting means into the second injection borehole.

The injection boreholes can be created simultaneously if desired,however it may be more economical that the second injection borehole iscreated after creating the first injection borehole, in correspondencewith extension of the cavity in the selected direction.

In order to reduce or prevent subsidence of the overburden formation,being the earth formation on top of the oil sand layer, the methodpreferably further comprises inserting a stream of refill material intothe cavity via the first injection borehole. Suitably, the stream ofrefill material comprises sand, for example cleaned sand transportedfrom the processing facility to the cavity.

To accommodate extension of the cavity in the selected direction, thelower section of the discharge borehole is suitably provided with aliner adapted to be changed in length, and wherein the method furthercomprises changing the length of the liner in correspondence withmovement of the front surface of the cavity in the selected direction.

If the lower section of the discharge borehole shortens as a result ofextending the cavity, the liner is suitably adapted to be shortened, andwherein the step of changing the length of the liner comprisesshortening the liner in correspondence with movement of the frontsurface of the cavity in the selected direction. For example, the stepof shortening the liner can comprise operating a cutting device to cutthe liner. To this end the liner is suitably made from a material thatcan be cut. The liner can be made of metals softer than steel, e.g.aluminium. Preferably the liner is made of a non-metal material, and inparticular the liner can be made of a plastics material. The liner canalso be shortened by the action of a fluid jet. Shortening can be doneby cutting or jetting away coarse discrete pieces of the liner, such asat suitable time intervals, or by producing small chips of the linermaterial.

Suitably, the discharge borehole is provided with pumping means forpumping the slurry via the discharge borehole to the processingfacility. Preferably the pumping means includes a pump sealed to aninner surface of the liner. In order to accommodate the change of lengthof the liner, it is preferred that the pump is axially movable throughthe discharge borehole, and that the method further comprises axiallymoving the pump through the discharge borehole in correspondence withchanging the length of the liner. Suitably the pump is driven by astream of fluid pumped through a conduit extending into the dischargeborehole. In such case, preferably at least a portion of said stream offluid is injected into the slurry of fluid and oil sand present in thecavity in order to stir the slurry in the cavity if desired.

The invention will be described hereinafter in more detail, and by wayof example, with reference to the accompanying drawings in which:

FIG. 1 schematically shows a system for use in an embodiment of themethod of the invention;

FIG. 2 schematically shows a detail of the system of FIG. 1;

FIG. 3 schematically shows the system of FIG. 1 during a further stageof the method of the invention;

FIG. 4 schematically shows a top view at surface of a layout using thesystem of FIG. 1; and

FIG. 5 schematically shows a top view at surface of another layout usingthe system of FIG. 1

In the Figures, like reference numerals relate to like components.

Referring to FIGS. 1 and 2 there is shown an earth formation containingan oil sands layer 2 located between an overburden layer 4 above the oilsand layer 2 and an underburden layer, shown as a layer of rock material6, such as limestone, below the oil sand layer 2. The oil sand layer 2has respective upper and lower boundaries 20, 22 extending horizontally.Thus, the layer of oil sand extends in an extension direction, inparticular a non-vertical extension direction, in this examplehorizontally between an overburden and an underburden. The layer of oilsand has a thickness defining a thickness direction, which is in thisexample vertical. The extension direction is different from, oftenperpendicular to, the thickness direction, and is in this example in thehorizontal plane. The layer extends more in the extension direction thatits thickness, typically for more than twice its thickness, such as formore that

or 10 times its thickness, or even more. It will typically extend forless than 10000 times its thickness. It will be understood that thelayer can extend along a plane.

A first injection borehole 8 extends vertically from a mobile injectionrig 10 at the earth surface 11 to a cavity 12 formed in the layer of oilsand 2. The cavity has an upper portion 12 a filled with air and a lowerportion 12 b containing a mixture (referred to hereinafter as “slurry”)13 of water and oil sand particles.

A deviated discharge borehole 14 extends from a production station 15 atthe earth surface to the cavity 12 whereby the production station 15 ishorizontally spaced from the mobile injection rig 10. The dischargeborehole 14 has an upper section 16 extending vertically and a lowersection 18 extending substantially parallel to the upper boundary 20and/or lower boundary 22 of the oil sands layer 2. Thus, in the presentexample the lower section 18 extends horizontally. Furthermore, thelower borehole section 18 extends in a selected direction within thelayer 2, in this example an azimuthal direction from the cavity 12, anddebouches into the lower cavity portion 12 b at some distance above thebottom of the cavity. The intersection between the lower boreholesection 18 and the cavity 12 defines a front surface 23 of the cavity12.

The selected direction suitably is a direction within the layer, inparticular a non-vertical direction, at an angle of less than 45 degreeswith an upper and/or lower boundary of the layer, preferablysubstantially parallel to the upper boundary and/or lower boundary ofthe layer. In particular the selected direction can be at least 45degrees away from the thickness direction, in particular at least 45degrees away from the vertical. Suitably the selected directionsubstantially coincides with the extension direction, so that the cavityis extended within the layer, substantially parallel with its upperand/or lower boundary, for more than the layer thickness, such as forbetween 2 and 5000 times the layer thickness. The expressionsubstantially parallel herein accounts for the precision with whichdeviated boreholes can be drilled in an underground layer.

The upper section 16 is provided with a conventional casing (or liner)24, whereas the lower section 18 is provided with a liner 26 of plasticsmaterial, for example glass fibre reinforced plastic, whereby the liner26 extends a minimal distance into the cavity 12. Furthermore, a jetpump 28 is positioned in the liner 26 in a manner that the jet pump 28is sealed relative to the inner surface of the liner 26 and is axiallymovable through liner 26. A fluid conduit 30 for driving the jet pump 28extends from the production station 15 through the casing 24 and theliner 26 to the jet pump 28. When driven by fluid pumped through thefluid conduit 30, the jet pump 28 is arranged to pump the slurry 13 offluid and particles from the lower cavity portion 12 b, via the annularspace between the fluid conduit 30 on one hand and the liner 26 andcasing 24 on the other hand, to the production station 15. The jet pump28 is thereto provided with one or more flow channels 32 (FIG. 2)allowing the slurry to flow in axial direction through the jet pump 28.The fluid conduit 30 extends further from the jet pump 28 through liner26 to the cavity 12. A lower end part 34 of the fluid conduit 30 isprovided with a cutter 36 for cutting the liner 26, one or more nozzles38 for initially forming the cavity 12 and for stirring the slurrypresent in the cavity 12, and a bit or mill 39 for crushing lumps ofrock material that may be present in the cavity 12.

An injection string 40 for injecting fluid into the cavity 12 extendsfrom the injection rig 10 via the first injection borehole 8 into thecavity 12, the injection string 40 having a lower end provided withjetting nozzles 42 located in the upper portion 12 a of cavity 12. Anannular seal 43 (such as a rotating head) is arranged in an upper partof the first injection borehole 8 to seal the annular space formedbetween the injection string 40 and the wall or casing of the firstinjection borehole 8.

Referring further to FIG. 3 there is shown the earth formation andseveral of the components shown in FIGS. 1 and 2 during a further stageof operation. The cavity 12 has been extended in the azimuthal directionof the lower borehole section 18 whereby the front surface 23 of thecavity has moved in said azimuthal direction. As a result, the lowerborehole section 18 has become shorter. The liner 26 has been shortenedat the side of the cavity 12 in correspondence with shortening of thelower borehole section 18. The fluid conduit 30 with the jet pump 28connected thereto has been pulled upward through discharge borehole 14over a distance about equal to the reduction in length of liner 26.Furthermore, a second injection borehole 44 extends vertically from amobile injection rig 46 to the cavity 12. The second injection borehole44 is spaced from the first injection borehole 8 in the azimuthaldirection. As is illustrated in FIG. 3, the lower end of the secondinjection borehole 44 is positioned closer to the front surface 23 ofcavity 12 than the lower end of the first injection borehole 8.

The injection string 40 has been removed from the first injectionborehole 8 and has been installed in the second injection borehole 44 soas to extend from the injection rig 46 into the cavity 12 whereby thejetting nozzles 42 again are positioned in the upper portion 12 a ofcavity 12. Alternatively another injection string, similar to injectionstring 40, can be applied in the second injection borehole 44. Anannular seal 47 (such as a rotating head) is arranged in an upper partof the second injection borehole 44 to seal the annular space formedbetween the injection string 40 and the wall or casing of the secondinjection borehole 44.

The first injection borehole 8 is now provided with a sand injectionstring 48 for inserting clean sand into the cavity 12. The sandinjection string 48 is suspended at surface by the mobile injection rig10 or by any other suitable means. A rear portion of the cavity 12 isfilled with a body of sand 49, which preferably includes a bindermaterial such as cement.

In this example the first and second injection boreholes and thedischarge borehole form a plurality of boreholes.

In the context of the present description, the assembly of cavity,discharge borehole, one or more injection boreholes, and productionstation is referred to as a “production unit”. In the example describedabove, the production unit includes two injection boreholes. However,depending on the stage of operation, the production unit can includeonly one, or more than two, injection boreholes. Generally, theproduction unit can include any suitable number of injection boreholesmutually spaced in the azimuthal direction.

Referring further to FIG. 4, there is schematically shown a top view atsurface of a layout of a plurality of production units 50, 52, 54, 56,58. Each production unit 50, 52, 54, 56, 58 is substantially similar tothe production unit described hereinbefore with reference to FIGS. 1-3,albeit that the number of injection boreholes varies per production unitdepending on the stage of operation. For ease of reference, therespective discharge boreholes and cavities are shown in dotted lines.

Production unit 50 includes discharge borehole 60, cavity 61, injectionboreholes 62, 63 and production station 64. Production unit 52 includesdischarge borehole 65, cavity 66, injection boreholes 67, 68 andproduction station 69. Production unit 54 includes discharge borehole70, cavity 71, injection boreholes 72, 73, 74 and production station 75.Production unit 56 includes discharge borehole 76, cavity 77, injectionboreholes 78, 79, 80 and production station 81. Production unit 58includes discharge borehole 82, cavity 83, injection boreholes 84, 85,86, 87 and production station 88. The cavities 61, 66, 71, 77, 83 haverespective front surfaces 90, 92, 94, 96, 98. Mobile injection rigs 100,102, 104, 106, 108, 110 are provided at surface, whereby injection rigs100, 102, 104, 106, 108 are in fluid communication with respectiveinjection boreholes 62, 67, 72, 78, 84, and whereby injection rig 110 isin fluid communication with each one of injection boreholes 68, 73, 74,79. The discharge boreholes 60, 65, 70, 76, 82 extend substantiallyparallel to each other at selected mutual horizontal spacings.Similarly, the cavities 61, 66, 71, 77, 83 extend substantially parallelto each other at selected horizontal spacings. The production stations64, 69, 75, 81, 88 are in fluid communication with a separation plant112 via a common pipeline 114 so as to allow pumping of the respectiveslurries of fluid and oil sand particles from the production units 50,52, 54, 56, 58 via the common pipeline 114 to the separation plant 112where hydrocarbon fluid is separated from the produced oil sandparticles.

Furthermore, reference numerals 116 relate to locations of injectionboreholes yet to be drilled at a further stage of operation, referencenumerals 118 relate to injection boreholes already drilled but not yetin fluid communication with the respective cavities 61, 66, 71, 77, 83,and reference numerals 120 relate to injection boreholes currently beingdrilled.

In the context of the present description, the assembly of productionunits 50, 52, 54, 56, 58 is referred to as a “field section”. The fieldsection described in the example above includes five production units,however it is to be understood that a field section can include anysuitable number of production units.

Referring further to FIG. 5 there is schematically shown a top view atsurface of an exemplary layout of a plurality of field sections 140,142, 144, 146 whereby field section 140 represents the assembly ofproduction units 50, 52, 54, 56, 58 described above. Field sections 142,144, 146 are substantially similar to field section 140 albeit these aremirrored relative to field section 140. Furthermore, field sections 140,142 are fluidly connected to separation plant 112 via common pipeline114, and field sections 144, 146 are fluidly connected to separationplant 112 via a common pipeline 148.

During normal operation of the system of FIGS. 1-3, the first injectionborehole 8 and the discharge borehole 14 are drilled into the oil sandslayer 2 using one or more conventional drilling rigs, and the casing 24and liner 26 are arranged in the discharge borehole 14. The mobileinjection rig 10 and the production station 15 are installed at theirrespective positions as indicated in FIG. 1. In a next step, the fluidconduit 30 with the jet pump 28 connected thereto is lowered through thedischarge borehole 14 until end part 34 of the fluid conduit 30 extendsjust beyond the far end of the liner 26. Water at high pressure is thenpumped from the production station 15 into the fluid conduit 30 so thatthe pumped water is jetted through the nozzles 38 to impact theformation at the end of the liner 26 with great force. If desired, thefluid conduit 30 is simultaneously rotated about its longitudinal axisto induce the bit or mill 39 to crush the rock formation. As a resultthe oil sand layer 2 is gradually excavated so that the cavity 12 andthe slurry 13 of water and oil sand particles are initially formed.Furthermore, by virtue of pumping of water through fluid conduit 30, thejet pump 28 is operated to pump the slurry 13 from the cavity 12 via theflow channels 32 of the jet pump 28 and via the annular space betweenthe fluid conduit 30 and the liner 26/casing 24, to the productionstation 15. In an alternative embodiment, the cavity 12 is initiated byunderreaming a lower portion of the first injection borehole 8 and/or alower portion of the discharge borehole 14 until fluid communicationbetween said boreholes 8, 14 is established, or by enlarging said lowerportion(s) in any other suitable manner.

Once the cavity 12 is sufficiently large so that fluid communicationbetween the cavity 12 and the injection borehole 8 is established, inparticular so that both boreholes 8,14 are intersecting the cavity 12,the injection string 40 is lowered into the first injection borehole 8,and water is pumped at high pressure from the injection rig 10 into theinjection string 40. The pumped water is jetted through the jettingnozzles 42 and impacts the wall of the cavity 12 with great force. As aresult, the oil sand layer 2 is further excavated and the slurry 13 ofwater and oil sand particles is continuously formed in the cavity 12.The size of the cavity increases as jetting of water through the nozzles42 continues. Water is pumped at a somewhat suitable lower pressure fromproduction station 15 into the fluid conduit 30 to operate the jet pump28. Thereby, the jet pump 28 pumps the slurry of fluid and particlesfrom the lower cavity portion 12 a, via the annular space between thefluid conduit 30 and the liner 26 or casing 24, and the flow channels 32of the jet pump 28, to the production station 15. Arrows 115 (FIGS. 1and 2) indicate the direction of flow of water pumped through fluidconduit 30, and arrows 116 indicate the direction of flow of the slurryof water and oil sand through said annular space and channels 32. Ifdesired, pumping of the slurry of fluid and particles from the cavity 12to the production station 15 can be enhanced by pressurising the cavity12 up to a few bars with a gas, such as compressed air or CO₂.Furthermore, a portion of the water pumped through fluid conduit 30 isjetted into the lower cavity portion 12 b through nozzles 38 in order toachieve some stirring of the slurry 13 in the cavity 12.

Jetting of water through nozzles 42 is continued so as to extend thecavity 12 in the azimuthal direction of the lower borehole section 18whereby the front surface 23 of the cavity moves in said azimuthaldirection. As a result, a portion 116 of the liner 26 (FIG. 2) graduallybecomes protruding into the cavity 12. In order to reduce the length of,or completely remove, the protruding liner portion 116, the cutter 36 isoperated to cut the protruding liner portion 116 at time intervalsselected in accordance with the speed of movement of the front surface23 in the azimuthal direction. The fluid conduit

is moved upwardly in correspondence with shortening of the liner 26whereby the jet pump 28 slides along the inner surface of the liner 26.If desired, the fluid conduit 30 is rotated to induce bit 39 to crushrock particles that may be present in the cavity 12.

The second injection borehole 44 is drilled into the oil sand layer 2before the front surface 23 of the cavity 12 reaches the location wherethe second injection borehole 44 intersects the cavity 12.Alternatively, the second injection borehole 44 can be drilled after thefront surface 23 of the cavity 12 reaches said location. Next, themobile injection rig 46 is installed, and the injection string 40 isremoved from the first injection borehole 8 and lowered into the secondinjection borehole 44. The cavity 12 is then further excavated in theazimuthal direction of the lower borehole section 18 in a manner similarto the manner described above with reference to the situation wherebythe injection string

extends through the first injection borehole 8. When the secondinjection borehole is drilled before the cavity 12 reaches the locationwhere the second injection borehole 44 intersects the cavity, fluidjetting can already be started via the second injection borehole whenthere is still a remaining wall with the cavity, but fluid communicationwith the cavity is already established. Powerful jetting action can leadto breaking through to the front surface 23 of the cavity 12 from thesecond injection borehole, removing the remaining wall.

Simultaneously with, or subsequent to, jetting of water into the cavity12 via the second injection borehole 44, sand is pumped into the rearportion of cavity 12 via the sand injection string 48 (FIG. 3). In thismanner, the rear portion of cavity 12 is gradually filled with the bodyof sand 49. In the context of the present description, any reference to“the cavity” is meant to include the upper cavity portion 12 a, thelower cavity portion 12 b, and the rear portion filled with sand.

In the above example, two injection boreholes 8, 44 have been describedfor excavating the cavity 12 in the desired azimuthal direction. Inpractice, any suitable number of injection boreholes can be appliedwhereby the injection boreholes are mutually spaced in the desiredazimuthal direction and whereby each pair of adjacent injectionboreholes is operated in a manner similar to operation of the injectionboreholes 8, 44 described above. However, in case more than twoinjection boreholes are applied, one or more of the injection boreholescan be positioned between the injection borehole instantaneously usedfor further excavating the cavity, and the injection borehole(s)instantaneously used for pumping sand into the cavity.

The slurry 13 of water and oil sand is transported from the productionstation 15 to a separation plant (not shown) for separating hydrocarbonfluid and sand particles from the slurry. Alternatively the productionstation 15 and the separation plant can be integrated in a single unit.Suitably, cleaned sand produced from the separation plant is used torefill the cavity 12 with the body of sand 49.

During normal operation of the system shown in FIG. 4, the dischargeborehole 82 and first injection borehole 87 of cavity 83 are drilledfirst whereafter production from cavity 83 is started. Next, thedischarge borehole 76 and first injection borehole 80 of cavity 77 aredrilled whereafter production from cavity 77 is started. Similarly, thedischarge boreholes 70, 65, 60 and first injection boreholes 74, 68, 63of respective cavities 71, 66, 61 are drilled in subsequent order, andproduction from these cavities also is started in subsequent order. As aresult, the degree to which the various cavities instantaneously extendin the azimuthal direction increases from, in subsequent order, cavity61, to cavity 66, to cavity 71, to cavity 77, to cavity 83.Alternatively, production from the various cavities can be startedsimultaneously or in any other suitable order, so that the cavitiesinstantaneously extend in the azimuthal direction to any suitabledegree.

Each production unit 50, 52, 54, 56, 58 is operated substantiallysimilar to normal operation of the production unit described withreference to FIGS. 1-3. Thus, the front surfaces 90, 92, 94, 96, 98 ofthe respective cavities 61, 66, 71, 77, 83 gradually move in theazimuthal direction of respective discharge boreholes 60, 65, 70, 76, 82as excavation of the cavities proceeds. The rear portions of thecavities are refilled with sand in correspondence with forward movementof the front surfaces. In the current situation, shown in FIG. 4, mobileinjection rig 110 is used to inject sand via injection boreholes 68, 73,74, 79 into the rear portions of respective cavities 66, 71, 77. Forthis purpose, mobile injection rig 110 is fluidly connected to injectionboreholes 68, 73, 79 via respective conduits 130, 132, 134. Injectionboreholes 80, 86 and 87 already have been used for sand injection,therefore these injection boreholes have been disconnected from mobileinjection rig 110. As illustrated in FIG. 4, the injection boreholes 118have been drilled before being in fluid communication with therespective cavities. In this manner, drilling of the injection boreholesdoes not delay excavation of the oil sand layer. Some water is separatedfrom the slurries of fluid and oil sand particles at the respectiveproduction stations 64, 69, 75, 81, 88. The slurries are then comingledin common pipeline 114 and transported to the separation plant 112 wherehydrocarbon fluid is separated from the comingled stream of water andoil sand particles.

Instead of excavating the cavities 61, 66, 71, 77, 83 such that theseare separated from each other by portions of rock material, as shown inFIG. 4, the cavities can be excavated so that such portions of rockmaterial vanish. In that case, two or more of the cavities areintegrated with each other so as to form a single large cavity.

During normal operation of the system shown in FIG. 5, each fieldsection 142, 144, 146 is operated in substantially the same manner asfield section 140 described with reference to FIG. 4. Thus, the slurriesof water and oil sand particles produced from the field sections 140,142, 144, 146 are transported to common separation plant 112 forseparation of hydrocarbon fluid from sand particles.

With the method described above it is achieved that hydrocarbon fluid isproduced from the oil sand layer without removing the overburden layer.Moreover, by refilling the cavities with sand it is achieved that anysubsidence of the overburden layer is reduced to a minimum. In apreferred embodiment, the cavities are refilled with sand from theproduced slurries of water and oil sand after cleaning at the separationplant. Suitably a binding material like cement is mixed into the sand.

In the examples described above, the discharge borehole is provided witha single pump (jet pump 28) for pumping the slurry of fluid and oil sandparticles via the discharge borehole to the production station atsurface. However, depending on the depth of the cavity and/or otheroperational parameters, a single pump may not suffice to pump the slurryto surface at an efficient flow rate. In that case, one or moreadditional pumps can be applied in the discharge borehole. For example,the upper section of the discharge borehole can be provided with asingle-stage or multi-stage centrifugal pump driven by a hydraulic orelectric motor, to pump the slurry to surface. Such centrifugal pump canbe positioned, for example, in the lower end part of the casing providedin the discharge borehole, just above the liner.

In the examples described above, the injection boreholes extend fromsurface locations mutually spaced in horizontal direction. In analternative arrangement, the injection boreholes extend as deviatedboreholes from a single surface location, or as branch boreholes of amultilateral borehole. Such arrangement can be attractive inapplications whereby the surface area is difficult accessible, forexample if the oil sand layer is located below a body of water or aswamp area.

1. A method of producing hydrocarbon fluid from a layer of oil sandlocated in an earth formation, the method comprising: creating aplurality of boreholes in the earth formation, including a firstinjection borehole and a second injection borehole spaced from the firstinjection borehole in a selected direction along which the layer of oilsand extends, and creating a cavity in the layer of oil sand, the cavitybeing in fluid communication with the first injection borehole;extending the cavity in the selected direction by operating fluidjetting means via the first injection borehole to jet a stream of fluidagainst a wall of the cavity; when the cavity is in fluid communicationwith the second injection borehole, operating the fluid jetting meansvia the second injection borehole to jet a stream of fluid against thewall of the cavity so as to further extend the cavity; and transportinga slurry of fluid and oil sand, resulting from operation of the fluidjetting means, from the cavity via a discharge borehole to a processingfacility for processing the slurry, the discharge borehole having alower section extending in the selected direction and being in fluidcommunication with the cavity at said wall of the cavity.
 2. The methodof claim 1, wherein the step of operating the fluid jetting means viathe second injection borehole comprises removing the fluid jetting meansfrom the first injection borehole and inserting the fluid jetting meansinto the second injection borehole.
 3. The method of claim 1, whereinthe second injection borehole is created after creating the firstinjection borehole, in correspondence with extension of the cavity inthe selected direction.
 4. The method of claim 1 wherein the methodfurther comprises inserting a stream of refill material into the cavityvia the first injection borehole.
 5. The method of claim 4, wherein thestream of refill material comprises sand.
 6. The method of claim 4wherein the stream of refill material is transported from the processingfacility to the cavity.
 7. The method of claim 1 wherein the lowersection of the discharge borehole is provided with a liner adapted to bechanged in length, and wherein the method further comprises changing thelength of the liner in correspondence with extension of the cavity inthe selected direction.
 8. The method of claim 7, wherein the liner isadapted to be shortened, and wherein the step of changing the length ofthe liner comprises shortening the liner in correspondence withextension of the cavity in the selected direction.
 9. The method ofclaim 8, wherein shortening the liner comprises operating a cuttingdevice to cut the liner, preferably wherein the liner is made of anon-metal material, more preferably of a plastics material.
 10. Themethod of claim 1, wherein the discharge borehole is provided withpumping means for pumping the slurry via the discharge borehole to theprocessing facility.
 11. The method of claim 10, wherein the pumpingmeans includes a pump sealed relative to an inner surface of the liner.12. The method of claim 10, wherein the pumping means is axially movablethrough the discharge borehole, and wherein the method further comprisesaxially moving the pumping means through the discharge borehole incorrespondence with changing the length of the liner.
 13. The method ofclaim 1 wherein the pumping means is driven by a stream of fluid pumpedthrough a conduit extending through the discharge borehole.
 14. Themethod of claim 13, wherein at least a portion of said stream of fluidis injected into the slurry of fluid and oil sand present in the cavity.15. (canceled)